Recently, with increased interest in environmental problems, a method for reducing a use amount of a raw material derived from a petroleum source has been studied in a variety of technical fields. A half or more of a total weight of a tire being generally sold now is constituted of a raw material being a petroleum source. For example, since a tire for a general passenger automobile contains such as about 20% by mass of a synthetic rubber, about 20% by mass of carbon black, a softening agent and a synthetic fiber, about 50% by mass or more of a whole tire is constituted of a raw material of the petroleum source. In addition, for the purpose Of improving processibility by improvement in stickiness and reduction in a viscosity, a stickiness imparting agent is blended into the rubber composition for a tire and, as this stickiness imparting agent, a resin derived from a petroleum source such as a C5-based resin, a C9-based resin, and a phenol-based resin is generally used. Then, development of a rubber for a tire using a raw material derived from a natural source satisfying the required properties equivalent to or exceeding those when a raw material derived from a petroleum source is used, is desired.
Here, for the purpose of providing a tread rubber composition satisfying industrial workability, and by which performance excellent in the gripping property is obtained, a tread rubber composition in which 100 to 150 parts by weight of a terpene-based resin is compounded based on 100 parts by weight of a diene-based rubber is known. In addition, the technique capable of providing a rubber composition for a tire, which is a substitute for a petroleum-based aroma oil, and gives a small load on the environment, by containing 50 parts by weight or more of a natural rubber or a polyisoprene rubber among diene-based rubber components, and compounding 1 to 50 parts by weight of a terpene resin having a specified dynamic viscosity, is also known. Further, for the similar object, a rubber composition for a tire tread containing 50 parts by weight or more of a natural rubber among diene-based rubber components, and containing a reinforcing filler containing 60% by weight or more of silica, and in which 0.5 to 15 parts by weight of a phenol-modified terpene resin having a specified softening point, a specified hydroxy value and a specified dynamic viscosity is compounded therein, is also known. However, since these rubber compositions reduce a load on the environment by compounding a large amount of a natural source material without compounding a petroleum-based resin, but a rubber composition for a side wall, and performance required by this are not considered at all, and processibility is not sufficient.
In addition, Japanese Patent Laying-Open No. 11-11106 (Patent Document 1) discloses a tubeless tire without an inner liner using a rubber composition for a case cord covering rubber layer, which contains 1 to 10 parts by weight of a stickiness imparting agent based on 100 parts by weight of a rubber component consisting of a diene-based rubber and bromide of an isobutylene/p-methylstyrene copolymer and, as this stickiness imparting agent, a terpene resin, and a rosin derivative are exemplified. According to Patent Document 1, it is described that adhesiveness with other tire member is excellent by using such the rubber composition.
In addition, Japanese Patent Laying-Open No. 2004-2584 (Patent Document 2) discloses a rubber composition in which 40 to 500 parts by weight of a filler and 15 parts by weight or more of a resin are compounded based on 100 parts by weight of a rubber component and, as this resin, a terpene resin and a rosin resin are exemplified. According to Patent Document 2, it is described that both of wet gripping performance and abrasion resistance can be realized by adopting such the composition, and using this in a tread part.
Further, Japanese Patent Laying-Open No. 2006-63093 (Patent Document 3) discloses a rubber composition for a tread, containing 8 parts by weight or more of a resin, and a filler consisting of 80% by weight or more of a white filler based on 100 parts by weight of a rubber component containing 80% by weight or more of a natural rubber and/or an epoxidized natural rubber and, as this resin, a terpene resin, an aromatic modified terpene resin, and a rosin resin are exemplified. According to Patent Document 3, it is described that a content of a source other than a petroleum can be enhanced and, at the same time, the previous performance can be similarly preserved.
However, the inventions disclosed in any of Patent Documents 1 to 3 do not consider a rubber composition for a side wall and performance required by this, and it is hard to say that processibility is sufficient.    Patent Document 1: Japanese Patent Laying-Open No. 11-11106    Patent Document 2: Japanese Patent Laying-Open No. 2004-2584    Patent Document 3: Japanese Patent Laying-Open No. 2006-63093